Primary explosives are sensitive explosive materials that are used, in relatively small quantities, to initiate a secondary or main explosive charge. Primary explosives are used in percussion primers and electric primers (hot-wire igniters) to initiate an explosion. Percussion primers contain a small amount of a sensitive ignition mix that is activated by a strike from a firing pin and are used for the direct ignition of a propellant powder as in small arms ammunition. Current non-corrosive percussion primer compositions consist of mixtures of lead styphnate, barium nitrate, calcium silicide, antimony sulphide and tetracene (1-amino-1-[(1H-tetrazol-5-yl)azo]guanidine hydrate) in varying amounts. Other primer compositions are known (e.g. FA70, FA90, etc.) and contain various other materials, but most widely used mixtures remain based on lead-containing components such as lead styphnate (e.g. FA956, 5086).
This well-known deflagrating material is undesirable from an environmental standpoint since its use and manufacture can contribute to or cause lead contamination. Tons of this toxic material are used annually in manufacture of military and commercial percussion primers and lead exposure via the primer mix itself or combustion products is common. The majority (>95%) of primer use in both the military and domestic police forces is for training purposes in “friendly” areas. A 1991 survey found that employees who had just cleaned a range operated by the FBI in Quantico, Va., had levels of lead in their blood almost 10 times as high as US government health limits. See Barsan, M. E. & Miller, A. Lead Health Hazard Evaluation HETA Report #91-0346-2572. As a result, both the EPA and the FBI have designated lead as a primer ingredient that requires replacement. Lead-free primer compositions currently in use contain diazodinitrophenol (DDNP). However, these mixes, due to concerns with shelf-life and reliability, do not meet requirements for use in military applications. See U.S. Pat. Nos. 5,417,160; 5,831,208; 6,478,903; and 7,056,401; and Publication No. WO/1999/144968. Alternatively, as described in U.S. Pat. No. 5,717,159, a new primer composition based on metastable interstitial composite (MIC) technology is known.
Hot-wire igniter systems (FIG. 1) are commonly used in both military and commercial applications as a method of initiation, wherein application of current from a power source is used to heat a filament and the heat is transferred to a reactive material (acceptor) to provide energy sufficient to ignite an output.
A hot-wire igniter is generally composed of a filament or bridgewire of high resistance, which is situated inside a composition that will ignite when a suitable current is applied. Common bridgewire materials are nichrome (tophet a or c) and stainless steel, which is composed of nickel, chromium and/or iron in various ratios. These materials have high heat resistance and will withstand high temperatures (˜1400° C.) before melting. They can therefore easily and rapidly transfer this heat to an ignitable composition such as a pyrotechnic or explosive charge.
Common materials that serve as acceptors of the bridgewire energy are lead styphnate, normal (“NLS”) or basic (“BLS”), and a number of pyrotechnics, such as ZPP (zirconium/potassium perchlorate). In these cases, the heat transferred from the bridgewire exceeds the ignition temperature of the acceptor and is sufficient to cause deflagration of that material. The energy of this deflagration may be used to further ignite a pyrotechnic, propellant or explosive output.